Experimental results of viscosity and density of mixtures of methane and n-decane are reported at temperatures from 303 to 393 K and pressures up to 75 MPa, at five compositions. This binary mixture is considered as a model for gas-condensate fluids since it presents dew-point and bubble point lines extending up to 35 MPa at room temperature. Viscosity and density were measured simultaneously using a vibrating-wire sensor, in an apparatus specifically built to study gas-liquid mixtures. The present results for both properties are compared with values from the literature when possible and present overall uncertainties of +/-0.2% for density and +/-3% for viscosity. Precisions are of about one third of the uncertainties for both properties. The excess volume of the mixture reaches -20 cm(3) mol(-1) at 393 K and 40 MPa. Viscosity has a marked non linear behaviour with composition, that is nonetheless well represented by a predictive scheme based on the theory of transport properties of the hard-sphere fluid. (C) 2003 Elsevier B.V. All rights reserved.